This invention relates to atomic force microscopes and, more particularly, in an atomic force microscope having a sensing tip mounted for vertical movement in response to relative horizontal movement of the sensing tip across a sample surface, to the method of manufacture and operation for sensing vertical movement of the sensing tip and for outputting a signal reflecting characteristics of the sample surface related to vertical movement of the sensing tip comprising the steps of, applying a reflective surface to move vertically in combination with the sensing tip; mounting a laser diode having light-emitting faces on opposite sides thereof close adjacent the sensing tip with one of the faces facing the reflective surface to emit a laser light beam therefrom and receive a portion of the light beam reflected from the reflective surface thereon; disposing a laser power detector close adjacent and facing the other of the faces of the laser diode to receive a combination of emitted and reflected laser light thereon and output an electrical signal directly related to the power of the laser light; scanning the sensing tip over the sample surface while operating the laser diode; and receiving the electrical signal from the laser power detector and using data therefrom to determine physical properties of the sample surface.
Atomic force microscopes are essentially surface profilometers which use very sharp tips and very low forces between the tip and sample. They can also operate in a mode in which the force is attractive and the tip does not touch the surface. A typical prior art atomic force microscope is described in U.S. Pat. No. 4,724,318. In this microscope a sharp tip on a flexible lever is held in contact with a surface. The height of the tip is detected by a tunneling microscope and this height measurement is used in a feedback loop to move the lever mount up and down to keep the bending of the lever, and therefore the force on the sample, constant. Subsequently, it has been found that the force exerted by the tunneling microscope on the atomic force tip is large and makes it difficult to operate the device with a low force between the force tip and sample. More suitable sensors of the tip height have been optical either interference or beam deflection. Y. Martin, et al, J. Appl. Phys. 61,4723, (1987) describes the former while O. Marti, B. Drake, and P. K. Hansma, Appl. Phys. Letters 51,484 (1987) describes the latter in the form of an atomic force microscope in which a beam of light is reflected off the back of the force tip. This typical prior art approach is depicted in FIG. 1. The sample 10 is mounted on the top of a piezoelectric scan tube 12. A scan tip 14 is mounted on the end of a cantilevered arm 16. A laser source 18 directs a laser light beam 20 onto the top of the arm 16 at the tip 14. The sample 10 is positioned under the tip 14 and the piezoelectric scan tube 12 is driven by a voltage which will move the sample 10 in a raster scan movement with respect to the tip 14. As the tip 14 moves up and down in its passages over the surface of the sample 10, the reflected light beam 20 is deflected and this deflection is measured by an optical detector 22. The foregoing system has the ability to measure tip motions with 1 angstrom resolution. Note that in this microscope, the tip cannot be scanned over the sample without scanning the entire optical system along with the tip. Due to the extended nature of the system, because the lever arms need to measure beam deflection, this is not practical; so, in all of the microscopes of this type the sample is scanned across the tip in the manner shown and described above. The sample, therefore, must be small in order to fit on the top of the scan tube. Usually, the samples are limited to being only a few millimeters square. Large samples, such as optical mirrors, cannot be measured with these microscopes, making these microscopes non-useful for many applications. The extended nature of the sensor (i.e. the distance between the source 18 and tip 14 and from the tip 14 to the detector 22) also makes it susceptible to thermal drifts of the structure.
Atomic force microscopes using optical interference have also been made; but, have been used only in a non-contact mode where the tip does not touch the surface. The tip is vibrated near its natural frequency and force gradients caused by the surface change the amplitude of oscillation. A microscope using an optical fiber and an interferometer to detect the position of the tip was described by Y. Martin, et al, J. Appl. Phys. 61,4723 (1987).
Wherefore, it is an object of this invention to provide an atomic force microscope using a laser diode and optical interference of light reflected back into the laser to measure the vertical position of the tip.
It is another object of this invention to provide an atomic force microscope using a laser diode and optical interference of light reflected back into the laser to measure the vertical position of the tip wherein the tip can be either off the surface and vibrated where changes in the amplitude of vibration near the natural frequency of the lever are used as a measure of changes of electric or magnetic force on the tip; or, the tip can be placed on the surface with no vibration to measure directly the profile of the surface.
It is still another object of this invention to provide a stand-alone atomic force microscope which can be placed on or suspended over the surface of large samples to scan local areas of the surface.
It is yet another object of this invention to provide a sensor for use in an atomic force microscope which sensor is small enough to be mounted in the scanner so that the sensor can be scanned over a sample and the sample can be stationary.
Other objects and benefits of the invention will become apparent from the detailed description which follows hereinafter when taken in conjunction with the drawing figures which accompany it.